Field of the Invention
This document relates to a touch sensing system, and more particularly, to a touch sensing system capable of touch input from a stylus pen and a driving method of the same.
Discussion of the Related Art
User interfaces (UI) enable humans (users) to easily control various types of electronic devices as they want. Typical examples of the user interfaces include keypads, keyboards, mice, on-screen displays (OSD), and remote controllers with an infrared communication capability or radio frequency (RF) communication capability. The user interface technology is continuing to make progress toward higher user sensitivity and ease of operation. Recently, user interfaces have been evolving into touch UI, voice recognition UI, 3D UI, etc.
The touch UI has been indispensably used in portable information appliances. The touch UI is implemented by building a touchscreen on the screen of a display device. Such a touchscreen can be implemented as a capacitive touchscreen. A touchscreen having capacitive touch sensors detects touch input by sensing a capacitance change, i.e., a change in the amount of electrical charge between the touch sensors when a finger or conductive object touches (or hovers over) a touch sensor.
The capacitive touch sensors can be implemented as self-capacitance sensors or mutual capacitance sensors. The electrodes of the self-capacitance sensors are connected to sensor lines oriented in one direction on a one-to-one basis. The mutual capacitance sensors are formed at the intersections of sensor lines Tx and Rx orthogonal to each other with a dielectric layer interposed between them.
Recently, stylus pens are widely used as a HID (human interface device), as well as fingers, in smartphones, smartbooks, etc. The stylus pens allows for more accurate input than fingers.
There are two types of stylus pens: active and passive. With the passive stylus, touch position detection is difficult because the changes in capacitance at contact points with the touchscreen are very subtle. With the active stylus, touch position detection is easier compared to the passive stylus because the active stylus itself generates and outputs a pen driving signal at a point where it hovers over or touches the touchscreen.
By the way, the related art active stylus pen uses a pen driving signal that has a different frequency than a touchscreen driving signal, in order to avoid interference with the touchscreen driving signal, as disclosed in Korean Patent Application Laid-Open Publication No. 10-2014-0043299 (also published as WO 2012/057888). In this technology, the touch module requires a sensing circuit for receiving the pen driving signal, which results in a rise in manufacturing costs.
One of the solutions suggested to overcome this problem is the technology that receives the touchscreen driving signal from the active stylus pen and then generates the pen driving signal in synchronization with the touchscreen driving signal to apply it the touchscreen.
The strength of the touchscreen driving signal received by the stylus pen decreases as the distance between the stylus pen and the touchscreen decreases, because of the air gap. That is, the strength of a first touchscreen driving signal received by the stylus pen while touching the touchscreen is less than the strength of a second touchscreen driving signal received by the stylus pen while hovering over the touchscreen.
In order to get a desired touch sensing signal, the pen driving signal needs to be synchronized with the touchscreen driving signal. However, if the strength of the touchscreen driving signal received by the stylus pen changes depending on the proximity between the touchscreen and the stylus pen, the synchronization is distorted. Once the synchronization between the pen driving signal and the touchscreen driving signal is distorted, the touch sensing signal becomes weaker, thus leading to low touch performance.